The preparation of acetic anhydride by contacting in the liquid phase a mixture comprising methyl acetate and/or dimethyl ether and methyl iodide with carbon monoxide in the presence of a carbonylation catalyst at elevated pressures and temperatures has been reported extensively in the patent literature. See, for example, U.S. Pat. Nos. 3,927,078; 4,046,807; 4,115,444; 4,252,741; 4,374,070; 4,430,273; 4,559,183; 5,003,104; 5,292,948 and 5,922,911 and European Patents 8396; 87,869; and 87,870. These patents disclose that the reaction rate can be increased if the catalyst system includes a promoter such as certain amines and quaternary ammonium compounds, phosphines and phosphonium compounds and inorganic compounds such as alkali metal salts e.g., lithium iodide. Normally, both the reaction (process) mixture and the crude product are substantially anhydrous, homogeneous liquids comprising a solution of the reactants and catalyst components in an inert solvent such as acetic acid. Thus, the crude, liquid product obtained from such acetic anhydride processes typically comprises a mixture of acetic anhydride and acetic acid as a result of the use of acetic acid as a process solvent.
Acetic acid may be coproduced in the process by feeding methanol and/or water to the production system, e.g., by feeding methanol and/or water to a process recycle stream containing acetic anhydride and/or to the carbonylation reactor. See, for example, U.S. Pat. Nos. 5,380,929, 6,130,355, EP-00087869-B1 and EP-00087870-B1. U.S. Pat. No. 4,374,070 discloses the possibility of adding methanol to an acetic anhydride-containing recycle stream.
The above-described processes for the manufacture of acetic anhydride are carried out by feeding carbon monoxide to a reaction zone containing a liquid mixture of (i) a feedstock compound selected from methyl acetate, dimethyl ether or a mixture thereof, (ii) methyl iodide and (iii) a carbonylation catalyst such as a Group VIII metal or metal compound and, optionally, one or more promoters. The carbon monoxide typically is fed below the surface of the liquid mixture in a finely divided form, e.g., by means of a gas sparging device, to maximize the concentration of carbon monoxide in the reaction mixture. Normally, the process is operated by feeding continuously the feedstock compound(s), methyl iodide, an inert solvent such as acetic acid, and catalyst or catalyst components dissolved in acetic anhydride and/or acetic acid and carbon monoxide to a reaction zone maintained at elevated temperature and pressure and removing continuously from the reaction zone a crude product mixture comprising acetic anhydride and acetic acid. The crude product mixture also contains feedstock compound(s), methyl iodide, acetic acid solvent, ethylidene diacetate, acetone, catalyst components and carbon monoxide dissolved in the crude product.
The crude product typically is fed continuously to a first separation zone wherein the pressure is reduced and the crude product flash distilled to produce (i) a vapor effluent comprising feedstock compound, methyl iodide, acetic acid solvent or product, acetic anhydride product, small amounts of by-product ethylidene diacetate and acetone and carbon monoxide and (ii) a liquid effluent comprising the catalyst or catalyst components dissolved in a mixture of acetic anhydride, acetic acid and small amounts of low boilers, e.g., feedstock compound, methyl iodide and acetone. The vapor effluent typically comprises about 20 to 60 weight percent of the crude product fed to the first separation zone. The liquid effluent is recycled to the reaction zone and the vapor effluent is fed to a product recovery zone wherein the acetic anhydride (and any coproduced acetic acid) is separated and removed from the production system. The other condensable components (methyl acetate, methyl iodide and acetic acid solvent) typically are recovered and recycled to the reaction zone. Co-product acetic acid may be recovered and removed from the process.
In the co-production of acetic acid and acetic anhydride, e.g., as described in U.S. Pat. Nos. 5,380,929 and 6,130,355, methanol and methyl acetate are fed to the primary carbonylation reactor and the products are produced in the presence of CO in a one-step reaction. In such processes, all of the heat of reaction is released in the single reaction step and there is no opportunity to utilize the heat of reaction directly in the production system such as in subsequent flash evaporation or distillation steps. The heat of reaction for methanol carbonylation is quite high, e.g., about twice that of the heat of reaction for methyl acetate carbonylation. Thus, a large amount of heat must be removed from the primary carbonylation reactor under severe conditions of high pressure and a corrosive environment. Furthermore, there is no opportunity to sequence the reactions in such a way as to obtain a beneficial shifting of reaction equilibrium.